EXPLANATION

There are two things to notice. First is that you're trying
to give new permissions to User2 and not change current permissions of
User2 within User1's mailbox. So we'll want to use Add instead of Set.
Set is used to change permissions if a user already has some sort of
permissions within the mailbox.

The second thing is
that the calendar is a folder within the mailbox. We don't want User2
to be able to read User1's emails, so we'll do
Add-MailboxFolderPermission instead of Add-MailboxPermission.